Reverse set with current limit for non-volatile storage

ABSTRACT

A storage system includes a substrate, control circuitry on the substrate, a three dimensional memory array (above the substrate) that includes a plurality of reversible resistance-switching elements, and circuits for limiting the SET current for the reversible resistance-switching elements. The memory cell is SET in a reverse biased fashion.

This application claims priority from provisional application61/076,553, filed on Jun. 27, 2008.

BACKGROUND

1. Field

The present invention relates to technology for data storage.

2. Description of the Related Art

A variety of materials show reversible resistance-switching behavior.These materials include chalcogenides, carbon polymers, perovskites, andcertain metal oxides and nitrides. Specifically, there are metal oxidesand nitrides which include only one metal and exhibit reliableresistance switching behavior. This group includes, for example, NiO,Nb₂O₅, TiO₂, HfO₂, Al₂O₃, MgOx, CrO₂, VO, BN, and AlN, as described byPagnia and Sotnick in “Bistable Switching in ElectroformedMetal-Insulator-Metal Device,” Phys. Stat. Sol. (A) 108, 11-65 (1988). Alayer of one of these materials may be formed in an initial state, forexample a relatively low-resistance state. Upon application ofsufficient voltage, the material switches to a stable high-resistancestate. This resistance switching is reversible such that subsequentapplication of an appropriate current or voltage can serve to return theresistance-switching material to a stable low-resistance state. Thisconversion can be repeated many times. For some materials, the initialstate is high-resistance rather than low-resistance.

These reversible resistance-switching materials are of interest for usein nonvolatile memory arrays. One resistance state may correspond to adata “0,” for example, while the other resistance state corresponds to adata “1.” Some of these materials may have more than two stableresistance states.

Non-volatile memories formed from reversible resistance-switchingelements are known. For example, U.S. Patent Application Publication2006/0250836, filed May 9, 2005 and titled “REWRITEABLE MEMORY CELLCOMPRISING A DIODE AND A RESISTANCE-SWITCHING MATERIAL,” which is herebyincorporated by reference herein in its entirety, describes arewriteable non-volatile memory cell that includes a diode coupled inseries with a reversible resistance-switching material such as a metaloxide or metal nitride.

However, operating memory devices that employ reversibleresistance-switching materials is difficult.

SUMMARY

A storage system is described that uses reversible resistance-switchingelements. Various circuits and methods are disclosed for controlling thesetting and resetting of the resistance for the reversibleresistance-switching elements.

One embodiment includes a substrate, control circuitry on the substrate,a three dimensional memory array (above the substrate) that includes aplurality of reversible resistance-switching elements, and circuits forlimiting the SET current for the reversible resistance-switchingelements. The memory cell is SET in a reverse biased fashion.

One embodiment includes a reversible resistance-switching memory cell, acurrent limiting circuit, a first control line in communication with thereversible resistance-switching memory cell, a first selection circuitin communication with the first control line, a second control line incommunication with the reversible resistance-switching memory cell, anda second selection circuit in communication with the second controlline. The first selection circuit selectively provides a first signal tothe reversible resistance-switching memory cell. The second selectioncircuit selectively connects the second control line to the currentlimiting circuit while the first selection circuit provides the firstsignal to reversible resistance-switching memory cell to provide areverse bias to the reversible resistance-switching memory cell thatwill set the reversible resistance-switching memory cell to a lowresistance state.

One embodiment includes a non-volatile memory cell, a first control linein communication with the memory cell, a first selection circuit incommunication with the first control line, a second control line incommunication with the memory cell, a second selection circuit incommunication with the second control line, and a current limitingcircuit in communication with the memory cell. The first selectioncircuit selectively provides a first signal to the memory cell. Thesecond selection circuit selectively provides a second signal to thememory cell while the first selection circuit provides the first signalto the memory cell to provide a reverse bias to the memory cell thatwill set the memory cell to a low resistance state.

One embodiment includes setting a reversible resistance-switchingnonvolatile storage element to a low resistance state by reverse biasingthe reversible resistance-switching nonvolatile storage element andlimiting current through the reversible resistance-switching nonvolatilestorage element using a current limiting circuit while setting thereversible resistance-switching nonvolatile storage element to the lowresistance state.

One embodiment includes setting a reversible resistance-switchingnonvolatile storage element to a low resistance state includingproviding a first signal to a first terminal of the reversibleresistance-switching nonvolatile storage element and providing a secondsignal to a second terminal of the reversible resistance-switchingnonvolatile storage element to cause a current through the reversibleresistance-switching nonvolatile storage element in a direction that isreverse of the easiest current flow through the reversibleresistance-switching nonvolatile storage element. The process alsoincludes limiting current through the reversible resistance-switchingnonvolatile storage element using a current limiting circuit whilesetting the reversible resistance-switching nonvolatile storage elementto the low resistance state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified prospective view of one embodiment of a memorycell with a reversible resistance-switching element.

FIG. 2 is a simplified prospective view of a portion of a first memorylevel formed from a plurality of the memory cells of FIG. 1.

FIG. 3 is a simplified prospective view of a portion of a threedimensional memory array.

FIG. 4 is a simplified prospective view of a portion of a threedimensional memory array.

FIG. 5 is a simplified prospective view of another embodiment of amemory cell with a reversible resistance-switching element.

FIG. 6 is a block diagram of one embodiment of a memory system.

FIG. 7 is a graph depicting I-V characteristics of a reversibleresistance-switching element.

FIG. 7A depicts a circuit that can read the state of a memory cell.

FIG. 8 is a graph depicting I-V characteristics of a diode in a logscale.

FIG. 9 is a graph depicting I-V characteristics of a reversibleresistance-switching element and a diode.

FIG. 10 is a schematic diagram of a circuit that can SET a memory cell.

FIG. 11 is a flow chart describing one embodiment of a process foroperating the circuit of FIG. 10.

FIG. 12 is a schematic diagram of a circuit that can SET a memory cell.

FIG. 13 is a schematic diagram of a circuit that can SET a memory cell.

FIG. 14 is a flow chart describing one embodiment of a process foroperating the circuit of FIG. 13.

FIG. 15 is a flow chart describing one embodiment of a process forrepeating the application of a SET voltage in order to SET a memorycell.

FIG. 16 is a schematic diagram of a circuit that can SET a memory cell.

FIG. 17 is a timing diagram that describes one embodiment of a processfor operating the circuit of FIG. 16.

FIG. 18 is a schematic diagram of a circuit that can SET a memory cell.

FIG. 18A is a flow chart describing one embodiment of a process foroperating the circuit of FIG. 18.

FIG. 19 is a schematic diagram of a circuit that can RESET a memorycell.

FIG. 20 is a flow chart describing one embodiment of a process foroperating the circuit of FIG. 19.

FIG. 21 is a schematic diagram of a circuit that can RESET a memorycell.

FIG. 21A is a flow chart describing one embodiment of a process foroperating the circuit of FIG. 21.

FIG. 22 depicts a voltage pulse applied to a reversibleresistance-switching element in order to SET the reversibleresistance-switching element.

FIG. 22A is a schematic diagram of a circuit that can SET a memory cell.

FIG. 23 is a schematic diagram of a circuit that can detect SET andRESET operations.

FIGS. 24A and 24B are flow charts describing embodiments of processesfor operating the circuit of FIG. 23.

DETAILED DESCRIPTION

A memory system is provided that includes memory cells with a reversibleresistivity-switching element. Various circuits and methods aredisclosed for controlling the setting and resetting of the resistancefor the reversible resistance-switching elements.

Memory Cell and System

FIG. 1 is a simplified perspective view of one embodiment of a memorycell 200 which includes a reversible resistance-switching element 202coupled in series with a steering element 204 between a first conductor206 and a second conductor 208.

Reversible resistance-switching element 202 includes reversibleresistivity-switching material 230 having a resistivity that may bereversibly switched between two or more states. For example, thereversible resistivity-switching material may be in an initialhigh-resistivity state upon fabrication that is switchable to alow-resistivity state upon application of a first voltage and/orcurrent. Application of a second voltage and/or current may return thereversible resistivity-switching material to the high-resistivity state.Alternatively, the reversible resistance-switching element may be in aninitial low-resistance state upon fabrication that is reversiblyswitchable to a high-resistance state upon application of theappropriate voltage(s) and/or current(s). When used in a memory cell,one resistance state may represent a binary “0” while another resistancestate may represent a binary “1.” However, more than two data/resistancestates may be used. Numerous reversible resistivity-switching materialsand operation of memory cells employing reversible resistance-switchingmaterials are described, for example, in U.S. Patent ApplicationPublication 2006/0250836, previously incorporated.

In one embodiment, the process of switching the resistance from thehigh-resistivity state to the low-resistivity state is referred to asSETTING the reversible resistance-switching element 202. The process ofswitching the resistance from the low-resistivity state to thehigh-resistivity state is referred to as RESETTING the reversibleresistance-switching element 202. The high-resistivity state isassociated with binary data “0” and the low-resistivity state isassociated with binary data “1.” In other embodiments, SETTING andRESETTING and/or the data encoding can be reversed.

In some embodiments, reversible resistance-switching material 230 may beformed from a metal oxide. Various different metal oxides can be used.In one example, nickel oxide is be used.

In at least one embodiment, through use of a selective depositionprocess, a nickel oxide layer may be used in a reversibleresistance-switching material without the nickel oxide layer beingetched. For example, a reversible resistance-switching element may beformed by employing a deposition process such as electroplating,electroless deposition, or the like, to selectively deposit anickel-containing layer only on conductive surfaces formed above asubstrate. In this manner, only the conductive surfaces on the substrateare patterned and/or etched (prior to deposition of thenickel-containing layer) and not the nickel-containing layer.

In at least one embodiment, the reversible resistance-switching material230 includes at least a portion of a nickel oxide layer formed byselectively depositing nickel and then oxidizing the nickel layer. Forexample, Ni, Ni_(x)P_(y) or another similar form of nickel may beselectively deposited using electroless deposition, electroplating or asimilar selective process, and then oxidized to form nickel oxide (e.g.,using rapid thermal oxidation or another oxidation process). In otherembodiments, nickel oxide itself may be selectively deposited. Forexample, an NiO-, NiO_(x)- or NiO_(x)P_(y)-containing layer may beselectively deposited above the steering element 204 using a selectivedeposition process and then annealed and/or oxidized (if necessary).

Other materials may be selectively deposited, and then annealed and/oroxidized if necessary, in accordance with the present invention to formreversible resistivity-switching materials for use in memory cells. Forexample, a layer of Nb, Ta, V, Al, Ti, Co, cobalt-nickel alloy, etc.,may be selectively deposited, such as by electroplating, and oxidized toform a reversible resistivity-switching material.

More information about fabricating a memory cell using reversibleresistance-switching material can be found in U.S. patent applicationSer. No. 11/772,084, “Memory Cell That Employs A Selectively DepositedReversible Resistance Switching Element and Methods of Forming TheSame,” filed on Jun. 29, 2007, incorporated herein by reference in itsentirety.

Reversible resistance-switching element 202 includes electrodes 232 and234. Electrode 232 is positioned between metal oxide reversibleresistivity-switching material 230 and conductor 208. In one embodiment,electrode 232 is made of platinum. Electrode 234 is positioned betweenmetal oxide reversible resistivity-switching material 230 and diode 204.In one embodiment, electrode 234 is made of Titanium Nitride, and servesas a barrier layer.

Steering element 204 can be a diode, or other suitable steering elementthat exhibits non-ohmic conduction by selectively limiting the voltageacross and/or the current flow through the reversibleresistance-switching element 202. In this manner, the memory cell 200may be used as part of a two or three dimensional memory array and datamay be written to and/or read from the memory cell 200 without affectingthe state of other memory cells in the array. Diode 204 may include anysuitable diode such as a vertical polycrystalline p-n or p-i-n diode,whether upward pointing with an n-region above a p-region of the diodeor downward pointing with a p-region above an n-region of the diode.

In some embodiments, diode 204 may be formed from a polycrystallinesemiconductor material such as polysilicon, a polycrystallinesilicon-germanium alloy, polygermanium or any other suitable material.For example, the diode 204 may include a heavily doped n+ polysiliconregion 242, a lightly doped or an intrinsic (unintentionally doped)polysilicon region 244 above the n+ polysilicon region 242, and aheavily doped p+ polysilicon region 246 above the intrinsic region 244.In some embodiments, a thin (e.g., a few hundred angstroms or less)germanium and/or silicon-germanium alloy layer (not shown), with about10% or more of germanium when using a silicon-germanium alloy layer, maybe formed on the n+ polysilicon region 242 to prevent and/or reducedopant migration from the n+ polysilicon region 242 into the intrinsicregion 244, as described, for example, in U.S. Patent ApplicationPublication No. 2006/0087005, filed Dec. 9, 2005 and titled “DEPOSITEDSEMICONDUCTOR STRUCTURE TO MINIMIZE N-TYPE DOPANT DIFFUSION AND METHODOF MAKING,” which is hereby incorporated by reference herein in itsentirety. It will be understood that the locations of the n+ and p+regions may be reversed.

When diode 204 is fabricated from deposited silicon (e.g., amorphous orpolycrystalline), a silicide layer may be formed on the diode to placethe deposited silicon in a low resistivity state, as fabricated. Such alow resistivity state allows for easier programming of the memory cellas a large voltage is not required to switch the deposited silicon to alow resistivity state.

As described in U.S. Pat. No. 7,176,064, “Memory Cell Comprising aSemiconductor Junction Diode Crystallized Adjacent to a Silicide,” whichis hereby incorporated by reference herein in its entirety,silicide-forming materials such as titanium and/or cobalt react withdeposited silicon during annealing to form a silicide layer. The latticespacing of titanium silicide and cobalt silicide are close to that ofsilicon, and it appears that such silicide layers may serve as“crystallization templates” or “seeds” for adjacent deposited silicon asthe deposited silicon crystallizes (e.g., the silicide layer enhancesthe crystalline structure of the silicon diode during annealing). Lowerresistivity silicon thereby is provided. Similar results may be achievedfor silicon-germanium alloy and/or germanium diodes.

Conductors 206 and 208 include any suitable conductive material such astungsten, any appropriate metal, heavily doped semiconductor material, aconductive silicide, a conductive silicide-germanide, a conductivegermanide, or the like. In the embodiment of FIG. 1, conductors 206 and208 are rail-shaped and extend in different directions (e.g.,substantially perpendicular to one another). Other conductor shapesand/or configurations may be used. In some embodiments, barrier layers,adhesion layers, antireflection coatings and/or the like (not shown) maybe used with conductors 206 and 208 to improve device performance and/oraid in device fabrication.

While the reversible resistance-switching element 202 is shown as beingpositioned above the steering element 204 in FIG. 1, it will beunderstood that in alternative embodiments, the reversibleresistance-switching element 202 may be positioned below the steeringelement 204.

FIG. 2 is a simplified perspective view of a portion of a first memorylevel 214 formed from a plurality of the memory cells 200 of FIG. 1. Forsimplicity, the reversible resistance-switching element 202, the diode204, and barrier layer 213 are not separately shown. The memory array214 is a “cross-point” array including a plurality of bit lines (secondconductors 208) and word lines (first conductors 206) to which multiplememory cells are coupled (as shown). Other memory array configurationsmay be used, as may multiple levels of memory.

FIG. 3 is a simplified perspective view of a portion of a monolithicthree dimensional array 216 that includes a first memory level 218positioned below a second memory level 220. In the embodiment of FIG. 3,each memory level 218 and 220 includes a plurality of memory cells 200in a cross-point array. It will be understood that additional layers(e.g., an inter-level dielectric) may be present between the first andsecond memory levels 218 and 220, but are not shown in FIG. 3 forsimplicity. Other memory array configurations may be used, as mayadditional levels of memory. In the embodiment of FIG. 3, all diodes may“point” in the same direction, such as upward or downward depending onwhether p-i-n diodes having a p-doped region on the bottom or top of thediode are employed, simplifying diode fabrication.

In some embodiments, the memory levels may be formed as described inU.S. Pat. No. 6,952,030, “High-Density Three-Dimensional Memory Cell,”which is hereby incorporated by reference herein in its entirety. Forinstance, the upper conductors of a first memory level may be used asthe lower conductors of a second memory level that is positioned abovethe first memory level as shown in FIG. 4. In such embodiments, thediodes on adjacent memory levels preferably point in oppositedirections, as described in U.S. patent application Ser. No. 11/692,151,filed Mar. 27, 2007 and titled “Large Array Of Upward Pointing P-I-NDiodes Having Large And Uniform Current,” which is hereby incorporatedby reference herein in its entirety. For example, the diodes of thefirst memory level 218 may be upward pointing diodes as indicated byarrow A₁ (e.g., with p regions at the bottom of the diodes), while thediodes of the second memory level 220 may be downward pointing diodes asindicated by arrow A₂ (e.g., with n regions at the bottom of thediodes), or vice versa.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as a wafer, withno intervening substrates. The layers forming one memory level aredeposited or grown directly over the layers of an existing level orlevels. In contrast, stacked memories have been constructed by formingmemory levels on separate substrates and adhering the memory levels atopeach other, as in Leedy, U.S. Pat. No. 5,915,167, “Three DimensionalStructure Memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

FIG. 5 shows a memory cell 250, which is a variation of memory cell 200of FIG. 1. Memory cell 250 differs from memory cell 200 of FIG. 1because the positions of electrodes 232 and 234 are swapped. That is,platinum electrode 232 is positioned between metal oxide reversibleresistivity-switching material 230 and diode 204, while Titanium Nitrideelectrode 234 is positioned between metal oxide reversibleresistivity-switching material 230 and conductor 208. It also differsfrom memory cell 200 of FIG. 1 because the location of n+ region 242 andp+ region 246 are reversed. A heavily doped n+ polysilicon region 242 isabove the intrinsic region 244 and a heavily doped p+ polysilicon region246 is below. This arrangement is useful to SET the reversibleresistance-switching element when the diode 204 is reverse biased asexplained in more detail below.

FIGS. 1-5 show memory cells in a cylindrical shape and conductors in theshapes of rails according to the disclosed arrangements. However, thetechnology described herein is not limited to any one specific structurefor a memory cell. Other structures can also be used to form memorycells that include reversible resistivity-switching material. Forexample, the following patents provide examples of structures of memorycells that can be adapted to use reversible resistivity-switchingmaterial: U.S. Pat. No. 6,952,043; U.S. Pat. No. 6,951,780; U.S. Pat.No. 6,034,882; U.S. Pat. No. 6,420,215; U.S. Pat. No. 6,525,953; andU.S. Pat. No. 7,081,377.

FIG. 6 is a block diagram that depicts one example of a memory system300 that can implement the technology described herein. Memory system300 includes a memory array 302 that can be a two or three dimensionalarray of memory cells as described above. In one embodiment, memoryarray 302 is a monolithic three dimensional memory array. The arrayterminal lines of memory array 302 include the various layer(s) of wordlines organized as rows, and the various layer(s) of bit lines organizedas columns. However, other orientations can also be implemented.

Memory system 300 includes row control circuitry 320, whose outputs 308are connected to respective word lines of the memory array 302. Rowcontrol circuitry 320 receives a group of M row address signals and oneor more various control signals from System Control Logic circuit 330,and typically may include such circuits as row decoders 322, arrayterminal drivers 324, and block select circuitry 326 for both read andprogramming (e.g., SET and RESET) operations. Memory system 300 alsoincludes column control circuitry 310 whose input/outputs 306 areconnected to respective bit lines of the memory array 302. Columncontrol circuitry 306 receives a group of N column address signals andone or more various control signals from System Control Logic 330, andtypically may include such circuits as column decoders 312, arrayterminal receivers or drivers 314, block select circuitry 316, as wellas read/write circuitry, and I/O multiplexers. System control logic 330receives data and commands from a host and provides output data to thehost. In other embodiments, system control logic 330 receives data andcommands from a separate controller circuit and provides output data tothat controller circuit, with the controller circuit communicating withthe host. System control logic 330 may include one or more statemachines, registers and other control logic for controlling theoperation of memory system 300.

In one embodiment, all of the components depicted in FIG. 6 are arrangedon a single integrated circuit. For example, system control logic 330,column control circuitry 310 and row control circuitry 320 are formed onthe surface of a substrate and memory array 302 is a monolithic threedimensional memory array formed above the substrate (and, therefore,above system control logic 330, column control circuitry 310 and rowcontrol circuitry 320). In some cases, a portion of the controlcircuitry can be formed on the same layers as some of the memory array.

Integrated circuits incorporating a memory array usually subdivide thearray into a number of sub-arrays or blocks. Blocks can be furthergrouped together into bays that contain, for example, 16, 32, or adifferent number of blocks. As frequently used, a sub-array is acontiguous group of memory cells having contiguous word and bit linesgenerally unbroken by decoders, drivers, sense amplifiers, andinput/output circuits. This is done for any of a variety of reasons. Forexample, the signal delays traversing down word lines and bit lineswhich arise from the resistance and the capacitance of such lines (i.e.,the RC delays) may be very significant in a large array. These RC delaysmay be reduced by subdividing a larger array into a group of smallersub-arrays so that the length of each word line and/or each bit line isreduced. As another example, the power associated with accessing a groupof memory cells may dictate an upper limit to the number of memory cellswhich may be accessed simultaneously during a given memory cycle.Consequently, a large memory array is frequently subdivided into smallersub-arrays to decrease the number of memory cells which aresimultaneously accessed. Nonetheless, for ease of description, an arraymay also be used synonymously with sub-array to refer to a contiguousgroup of memory cells having contiguous word and bit lines generallyunbroken by decoders, drivers, sense amplifiers, and input/outputcircuits. An integrated circuit may include one or more than one memoryarray.

SET with Current Limiting

As described above, reversible resistance-switching element 202 may bereversibly switched between two or more states. For example, thereversible resistivity-switching material may be in an initial,high-resistivity state upon fabrication that is switchable to alow-resistivity state upon application of a first voltage and/orcurrent. Application of a second voltage and/or current may return thereversible resistivity-switching material to a high-resistivity state.FIG. 7 is a graph of voltage versus current for one example embodimentof a metal oxide reversible resistance-switching element. Line 400represents the I-V characteristics of the reversibleresistance-switching element when in the high-resistivity state(R_(OFF)). Line 402 represents the I-V characteristics of the reversibleresistance-switching element when in the low-resistivity state (R_(ON)).

To determine which state the reversible resistance-switching element isin, a voltage is applied and the resulting current is measured. A highermeasured current (see line 402) indicates that the reversibleresistance-switching element is in the low-resistivity state. A lowermeasured current (see line 400) indicates that the reversibleresistance-switching element is in the high-resistivity state. Note thatother variations of a reversible resistance-switching element havingdifferent I-V characteristics can also be used with the technologyherein.

FIG. 7A depicts a circuit that illustrates one embodiment for readingthe state of a memory cell. FIG. 7A shows a portion of a memory arrayincluding memory cells 450, 452, 454 and 456, all of which are based onthe embodiments of FIGS. 1-5. Two of the many bit lines and two of themany word lines are depicted. A read circuit for one of the bit lines isdepicted to be connected to the bit line via transistor 458, which iscontrolled by a gate voltage supplied by column decoder 312 in order toselect or unselect the corresponding bit line. Transistor 458 connectsthe bit line to a Data bus. Write circuit 460 (which is part of systemcontrol logic 330) is connected to the Data bus. Transistor 462 connectsto the Data bus and operates as a clamp device that is controlled byclamp control circuit 464 (which is part of system control logic 330).Transistor 462 is also connected to comparator 466 and reference currentsupply Iref. The output of comparator 466 is connected to a data outterminal (to system control logic 330, a controller and/or a host) andto data latch 468. Write circuit 460 is also connected to data latch468.

When attempting to read the state of the reversible resistance-switchingelement, all word lines are first biased at Vread (e.g., approximately 2volts) and all bit lines are at ground. The selected word line is thenpulled to ground. For example purposes, this discussion will assume thatmemory cell 450 is selected for reading. One or more selected bit linesare pulled to Vread through the data bus (by turning on transistor 458)and the clamp device (transistor 462, which receives ˜2 volts+Vt). Theclamp device's gate is above Vread but controlled to keep the bit linenear Vread. Current is pulled by the selected memory cell throughtransistor 462 from the Vsense node. The Vsense node also receives areference current Iref that is between a high-resistivity state currentand a low-resistivity state current. The Vsense node moves correspondingto the current difference between the cell current and the referencecurrent Iref. Comparator 466 generates a data out signal by comparingthe Vsense voltage to a Vref-read voltage. If the memory cell current islarger than Iref, the memory cell is in the low-resistivity state andthe voltage at Vsense will be lower than Vref. If the memory cellcurrent is smaller than Iref, the memory cell is in the high-resistivitystate and the voltage at Vsense will be higher than Vref. The data outsignal from comparator 466 is latched in data latch 468.

Looking back at FIG. 7, while in the high-resistivity state (see line400), if the voltage VSET and sufficient current is applied, thereversible resistance-switching element will be SET to thelow-resistivity state. Line 404 shows the behavior when VSET is applied.The voltage will remain somewhat constant and the current will increasetoward Iset_limit. At some point, the reversible resistance-switchingelement will be SET and the device behavior will be based on line 402.Note that the first time the reversible resistance-switching element isSET, Vf (the forming voltage) is needed to SET the device. After that,VSET can be used. The forming voltage Vf may be greater than VSET.

While in the low-resistivity state (see line 402), if the voltage VRESETand sufficient current (Ireset) is applied, the reversibleresistance-switching element will be RESET to the high-resistivitystate. Line 406 shows the behavior when VRESET is applied. At somepoint, the reversible resistance-switching element will be RESET and thedevice behavior will be based on line 400.

In one embodiment, Vset is approximately 5 volts, Vreset isapproximately 3 volts, Iset_limit is approximately 5 uA and the Iresetcurrent could be as high as 30 uA.

If the current gets too high during a SET operation, it is possible forthe reversible resistance-switching element to SET and then immediatelyRESET due to the high current. In some cases, the reversibleresistance-switching element will oscillate between SET and RESET. Otherunpredictable behavior may also occur. To prevent such a situation,technology is proposed herein for limiting the current during a SEToperation in such a manner that the current can be as high asIset_limit, but not high enough to cause an immediate RESET oroscillation.

One proposal for limiting the current during a SET operation is to SETthe reversible resistance-switching element through a diode that isreverse biased. For example, looking at FIG. 5, it is proposed thatdiode 204 be reversed biased during a SET operation. That means that ahigher voltage will be applied to conductor 208 then to conductor 206 tocreate the reverse bias between the p+ region 242 and the TitaniumNitride electrode 234. Because the diode is reverse biased, the currentthrough the diode, and therefore the current through the reversibleresistance-switching element, will be limited. In this embodiment, thediode is to be forward biased when the reversible resistance-switchingelement is RESET. This SET operation could also be used with the memorycell 200 in FIG. 1 as well as other cell structures by applying voltagepolarities at the conductors which achieve the same polarity on thediode and the resistive switching element.

FIG. 8 shows the I-V characteristics (on a log scale) for diode 204. Inthe positive voltage range (forward bias), represented by the right sideof the graph, the current quickly increases as the voltage increase. Inthe negative voltage range (reverse bias), the current increase is muchlower until breakdown. A large current at reverse bias can damage thediode. Reverse bias is applied through a current limiting circuit thatlimits the current to prevent damage to the diode. The same currentlimit provides the before mentioned Iset_limit desired for the format orset operations.

In one embodiment, the diode is designed to have a low reverse softbreakdown voltage. Such a design can be accomplished by limiting thethickness of the region between the n+ and p+ regions.

FIG. 9 is a graph of voltage versus current for the metal oxidereversible resistance-switching element and the diode. Lines 400-406 areas discussed above. Line 420 represents the I-V characteristics of thediode during reverse bias. Line 422 shows the I-V characteristics of thediode at the breakdown voltage Vbd. Because the diode and the reversibleresistance-switching element are connected in series, they willexperience the same current. The device which has the lowest currentwill limit the current for the other device. As such, during forwardbias, the memory cell comprising the diode and the reversibleresistance-switching element will operate based on lines 400, 402 and406. Resetting will be done by applying VRESET when in thelow-resistivity state. When it is desired to SET the memory cell, thememory cell will be reverse biased and the memory cell will operatebased on line 420 and line 422. When a voltage potential of Vset (e.g.,−Vset) is applied across the reversible resistance-switching element,the current will try to rise. As the current increases, the reversibleresistance-switching element will be SET. Because the diode is reversebiased, the current increase will be limited by the diode reversecurrent in soft breakdown, thereby, preventing an immediate RESET oroscillation between SET and RESET.

FIG. 10 is a schematic diagram of a circuit for SETTING a memory cell.FIG. 10 shows four memory cells 500, 502, 504 and 506, each of whichincludes a diode and a reversible resistance-switching element. In afull array, there would be much more than four memory cells. In oneembodiment, the memory cells are based on the embodiment of FIG. 5. Inanother embodiment, the memory cell of FIG. 1 can be used. Either way,the structures of FIG. 2, 3 or 4 can be used.

Memory cell 500 of FIG. 10 is selected for setting, as it is at theintersection of the selected word line and the selected bit line. Eachword line will have a driver circuit, represented by transistors 510 and512 connected between VPP and ½ VPP. In one embodiment, VPP(approximately 6-10 volts) is the highest voltage available on theintegrated circuit. By applying 0 volts to the gates of transistors 510and 512, VPP will be driven on a selected word line. By applying VPP tothe gates of transistors 510 and 512, ½ VPP will be driven on anunselected word line. If a bias near ground is applied to the selectedbit line and VPP is applied to the selected word line, the diode ofmemory cell 500 will be reverse biased beyond the reverse breakdownvoltage of the diode and the selected cell can be SET. If a bias nearground is applied to the selected bit line and ½ VPP is applied to aword line, there will not be a sufficient voltage differential to SETthe memory cell.

A BL Selection Circuit comprises connected transistors 520 and 522.There will be one BL Selection Circuit for each bit line or a set of BLSelection Circuits that can be switchably connected to different subsetsof bit lines. If 0 volts is applied to the gate of transistors 520 and522, then ½ VPP is driven on an unselected bit line. For a selected bitline, ½ VPP is applied to the gate of transistors 520 and 522 so thatthe bit line is pulled to a bias near ground by node 521 and current(representing the current through the selected memory cell) is passed tonode 521.

Node 521 is connected to a current mirror that comprises transistors 524and 526 connected at their gates. Another circuit (not depicted in FIG.10) supplies a reference current I_(LIMREF). In one embodiment,I_(LIMREF) is equal to Iset_limit. In another embodiment, I_(LIMREF) isindicative of Iset_limit. The current I_(SET), which flows throughtransistor 526, will mirror I_(LIMREF). If the current at node 521approaches I_(SET), the voltage at node 521 (labeled as VSENSE) willincrease. The voltage VSENSE is provided to comparator 530, whichcompares VSENSE to V_(REF). When VSENSE equals V_(REF), the output ofcomparator 530 will indicate that a SET operation has been detected. Thereference voltage V_(REF) is set so that it represents a value of VSENSEthat corresponds to the memory cell current through device 522 beingequal to (or slightly above) Iset_limit. This circuit assumes that thecurrent will approach Iset_limit when the memory cell SETs; therefore,this condition is detected by comparator 530. The output of comparator530 is used to disable the circuit that generates I_(LIMREF) andunselect the bit line by providing a signal to the gate of transistor533 in order to force ½ VPP on to the bit line.

FIG. 11 is a flow chart describing the behavior of the circuit of FIG.10 during a SET operation. In step 550, all word lines and all bit linesare biased at ½ VPP. In step 552, the selected word line is biased atVPP, for example, by applying 0 volts to the gate of transistors 510 and512. The voltage VPP is sufficient to cause a reverse current throughthe diode of one uA or more and still have a voltage across the resistormaterial of almost 2 volts. In another embodiment, the selected wordline is biased to a voltage that is at least a diode drop higher thanthe voltage on the unselected word lines. In step 554, the BL SelectionCircuit connects the selected BL to the current limiter circuit (currentmirror and comparator 530) with a path to ground. As such, the selectedbit line falls sufficiently to provide a sufficient voltage differentialthat will SET the reversible resistance-switching element of theselected memory cell. In step 556, the bit line voltage rises when theSET occurs, due to the current-limit circuit. In step 558, comparator530 detects that VSENSE has risen to the Vref, thereby detecting the SEToperation. In step 560, the output of comparator 530 is used to disablethe generation of I_(LIMREF) and to apply the “save” voltage of ½ VPP tothe bit line to prevent the memory cell from being over-SET (e.g.,causing an immediate RESET or oscillation between RESET and SET). Theprocess of FIG. 11 can be performed for one memory cell or in parallelfor multiple memory cells. Another embodiment includes an implementationwhere the selected word line is at ground and the selected BL has a pathto a voltage at least a diode drop above the ½ VPP.

FIG. 12 is a schematic diagram of a second embodiment of a circuit forsetting a memory cell. The difference between the circuits of FIG. 12and FIG. 10 is that the circuit of FIG. 12 uses triple well technology.That is, by placing an nmos transistor in a p-well (where the p-well isin an n-well that is in a p-substrate), negative voltages can be used.The use of negative voltages allows all voltages to be reduced by ½ VPP.This arrangement saves power and causes less stress on the circuit.

In one embodiment, the memory cells can be read before performing a SEToperating. Then, only those memory cells that are supposed to be SET andthat are in the high-resistivity state will be SET. Memory cells thatare supposed to be SET, but are in the low-resistivity state will notneed to be SET.

FIG. 12 shows four memory cells 570, 572, 574 and 576, each of whichincludes a diode and a reversible resistance-switching element. Memorycell 570 is selected for setting, as it is at the intersection of theselected word line and the selected bit line. Each word line will have adriver circuit, represented by transistors 580 and 582 connected between½ VPP and ground. By applying 0 volts to the gate of transistors 510 and512, ½ VPP will be driven on a selected word line. By applying ½ VPP tothe gate of transistors 580 and 582, 0 volts will be driven onunselected word lines. If a bias near −½ VPP volts is applied to theselected bit line and ½ VPP is applied to the selected word line, thediode of memory cell 570 will be reverse biased beyond its reversebreakdown voltage and the cell 570 will be SET. If a bias near −½ VPPvolts is applied to the selected bit line and 0 volts is applied to aword line, there will not be a sufficient voltage differential to SETthe memory cell.

The BL Selection Circuit comprises connected transistors 584 and 586.There will be one BL Selection Circuit for each bit line or a set of BLSelection Circuits that can be switchably connected to different subsetsof bit lines. If −½ VPP is applied to the gate of transistors 584 and586, then 0 volts is driven on an unselected bit line. For a selectedbit line, 0 volts is applied to the gate of transistors 584 and 586 sothat the bit line is pulled to a bias near −½ VPP by device 590 andcurrent (representing the current through the selected memory cell) ispassed to the current limiting circuit.

Transistor 586 is connected to a current mirror that comprisestransistors 588 and 590 connected at their gates. Another circuit (notdepicted in FIG. 12) supplies a reference current I_(LIMREF). If thecurrent out of transistor 586 approaches I_(SET), the voltage at node521 (labeled as VSENSE) will increase. The voltage VSENSE is provided tocomparator 594, which compares VSENSE to V_(REF). When VSENSE equalsV_(REF), the output of comparator 594 will indicate that a SET operationhas been detected, the generation of the reference current I_(LIMREF)will be disabled and the bit line will be pulled to ground.

The circuit of FIG. 12 operates similar to the circuit of FIG. 10, withdifferent voltage levels being used (as noted above). As such, the flowchart of FIG. 11 applies to the circuit of FIG. 12, with some changes involtages For example, in step 550 the word lines and bit lines arebiased at 0 volts. In step 552, the selected word line is biased at ½VPP. In step 554, the bit line is connected to the current limitercircuit with a path to −½ VPP. The voltage across the selected memorycells is VPP (−½ VPP to +½ VPP).

SET Using Capacitive Discharge

In some embodiments, circuits that provide, control and/or limit thecurrent through a memory cell can be far away from the memory cell. Thisdistance can be more of an issue for a monolithic three dimensionalmemory array where the control circuitry is on the substrate surface andthe memory cells are on upper layers of the three dimensional memoryarray (as described above). Because of this distance, the conductivepaths can get quite long which results in relatively large capacitancesfor the lines. In some cases, after a memory cell is SET, the capacitivecharge on the lines will subsequently dissipate through the memory cell,which can cause extra current to pass through the reversibleresistance-switching element. This extra current may cause thereversible resistance-switching element to SET to such a low resistancevalue that it is difficult or impossible to RESET the element. Oneproposed solution is to discharge the bit line and data bus during theSET operation so that after the SET have been achieved, no unwantedcurrent will subsequently be driven through the memory cell. In thisembodiment, the diode will be forward biased during the SET operationand Vset will be applied as a pulse. The Vset pulse will be shorter thenthe time needed to SET the reversible resistance-switching element sothat the charge from the bit line and data bus will be needed to providethe extra charge not provided by the Vset pulse. In someimplementations, the SET operation can be followed by a verify operationto see if the SET operation was successful. If not, the SET operationcan be retried.

FIG. 13 is a schematic diagram of one embodiment of a circuit that canbe used to set a memory cell using the capacitive discharge describedabove. In some embodiments, there will be one such circuit for each bitline or a group of such circuits that can be selectively connected todifferent groups of bit lines.

The circuit of FIG. 13 includes a memory cell 602 that comprises areversible resistance-switching element and a diode, as described abovewith respect to FIGS. 1-5. Memory cell 602 is connected to a bit line BLhaving a capacitor 604. In one embodiment, capacitor 604 is about 1 pf.The bit line BL is connected to a Data bus via the BL Selection Circuit.In one embodiment, the each bit line has its own BL selection circuit,and each bit line has its own Data bus line. The control circuitry forthe memory system sends column selection signals CSG<15:0> and XCQ<3:0>to the various BL Selection Circuits to identify which bit lines shouldconnect to the Data bus. An appropriate one of the signals CSG<15:0> isprovided to the input of inverter 614 and an appropriate one of thesignals XCQ<3:0> is provided to the power pin of inverter 614 so thatthe output XCSEL of inverter 614 will be 0 volts when the associated bitline BL is selected; otherwise, XCSEL of inverter 614 will be VPP. Thesignal XCSEL is provided to the gates of transistors 610 and 612. WhenXCSEL of inverter 614 is at VPP, the unselected bit line voltage UBL of0.7 volts (approximately one diode drop) is provided to the bit line viatransistor 612. When XCSEL of inverter 614 is at 0 volts, the Data busis connected to the bit line via transistor 610. The Data bus, whichincludes parasitic capacitance 608, is connected to transistor 606. Thegate of transistor 606 receives a pulse. Between pulses, the Data bus isfloating. During the pulse (a negative pulse), VPP is provided to theData bus (via transistor 606) to charge-up the data bus parasiticcapacitance 608. When the BL selection circuit is selected, the chargefrom the Data bus charges the bit line BL and its capacitance 604. Whenthe path to VPP is shut off, the bit line is floating and the charge onthe bit line BL (and capacitor 604) will discharge though memory cell602. In one embodiment, the diode is forward biased and only positivevoltages are used.

FIG. 14 is a flow chart of one embodiment of a process for operating thecircuit of FIG. 13. The process of FIG. 14 can be performed on onememory cell or on multiple memory cells concurrently. In step 630, theselected word line is pulled to ground. The unselected word lines are atVPP-0.7v. In step 632, the selected bit line is pulled to VPP. This canbe accomplished in tens of nano seconds by applying the depicted pulse(XSA_ENABLE) to the gate of transistor 606 and the appropriate selectionsignals CSG<15:0> and XCQ<3:0>. Unselected bit lines are at 0.7 volts.In step 634, the path to VPP is shut off due to the pulse (XSA_ENABLE)being over. Thus, the data bus and the bit line are floating. While thebit line was at VPP in step 634, the reversible resistance-switchingelement of the memory cell was receiving a sufficient voltage to performa SET operation. However, the duration of the application of VPP was notlong enough to cause a SET. In one embodiment, the reversibleresistance-switching element needs hundreds of nano seconds to SET;however, VPP is only provided for tens of nano seconds. Because the pathto VPP is shut off, in step 636 the bit line capacitance (and in someembodiments, depending on operation of the selection signals, the databus capacitance) is dissipated through the memory cell, including thereversible resistance-switching element. The extra charge fromdissipating the capacitive charge may be enough to finish the SEToperation.

In some embodiments, it is possible that the extra charge fromdissipating the capacitive charge is not enough to finish the SEToperation. Therefore, in some implementations, the process of FIG. 15 isused to perform a SET of a memory cell. In step 650 of FIG. 15, theprocess of FIG. 14 is performed. In step 652, a verify operation isperformed to see if the memory cell is SET. In one embodiment, a readvoltage (less than Vreset) is applied. Based on the current sensedthrough the memory cell, the control circuitry determines whether thereversible resistance-switching element is in a high-resistivity stateor low-resistivity state. If the memory cell verifies to be in thelow-resistivity state (see step 654), then in step 656 the memory cellis unselected from the SET process. If the memory cell does not verifyto be in the low-resistivity state (see step 654), then the processloops back to step 650 and repeats. Note that the process of FIG. 15 canbe used with the other procedures described herein to SET or RESET amemory cell.

The capacitive discharge method described above limits the maximumelectrical charge that flows through the memory cell in the SEToperation. The maximum electrical charge in the SET is dependent on thevoltage on the bit line applied before the SET and the capacitance onthe bit line (and optionally the data bus which is connected to the bitline). The maximum electrical charge is insensitive to the resistance ofthe diode in the memory cell. This leads to a higher Ron after the SEToperation. A higher Ron leads to a lower Ireset, the current required toreset the reversible resistance-switching element. The diode can providethat Ireset because the bit line is held at a sufficient voltage duringthe RESET operation.

As described above, the selected bit line is charged and isolated byturning on and off a pre-charge device (transistor 606) connected to thedata bus which is thereby connected to the selected bit line. Anotherimprovement to the method of FIG. 14 is to detect the increase incurrent through the memory cell when it SETs and using that detecting todeselect the bit line. The column decoder circuitry then pulls the bitline down to a deselected level more quickly than the discharge thoughthe cell further reducing the time current flows through the cell.

FIG. 16 is a schematic diagram of another embodiment of a circuit thatcan be used to set a memory cell using the capacitive dischargedescribed above. In some embodiments, there will be one such circuit foreach bit line or a group of such circuits that can be selectivelyconnected to different groups of bit lines.

In some embodiments, it is desirable to select the word line firstbecause in some monolithic three dimensional memory arrays the word lineselection is slow. The charge may be placed on the bit line capacitancevery quickly by charge sharing as illustrated in FIG. 16. An additionalcapacitor is charged to the highest voltage available in the circuitryduring a pre-charge time. Then, the bit line is selected and a chargesharing device 710 is turned on to connect this capacitor to the bitline. The connected capacitors quickly reach a desired voltage for theSET operation determined by the capacitance ratio, and then the chargesharing device is shut off. The SET operation occurs after the bit linereceives the charge transfer because it takes a longer time to SET thereversible resistance-switching element than to transfer the charge.

The circuit of FIG. 16 includes a memory cell 702 that comprises areversible resistance-switching element and a diode, as described abovewith respect to FIGS. 1-5. Memory cell 702 is connected to a bit line BLhaving a capacitance 704. In one embodiment, capacitance 704 is 1 pf.The bit line BL is connected to a Data bus via the BL Selection Circuit.In one embodiment, the each bit line has its own BL selection circuit,and many Bit lines can be connected to a multi-line Data Bus. The BLselection circuit of FIG. 16 is identical to the bit line selectioncircuit of FIG. 13.

The Data bus is connected to the bit line via transistor 610. The Databus, which comprises capacitance 712 (e.g. 2 pf), is connected totransistor 710 which controls the charge sharing. The gate of transistor710 receives a pulse (XPG_PULSE). Between pulses, the Data bus (nodeSELB) is floating and isolated from node GSELB. During the pulse (anegative pulse), the data bus (node SELB) is connected to GSELB.Capacitor 708 (e.g. 0.5 pf) is connected to ground from GSELB.

Transistor 706, which is connected to VPP and to GSELB, receives a pulse(XSA_ENABLE). Between pulses, GSELB is floating. During a negativepulse, VPP is used to charge GSELB without a current limit. Whentransistor 710 receives a pulse at its gate, the charge at GSELB is usedto charge SELB to (VPP)×(Capacitance of Data bus)/(Capacitance of Databus+Capacitance of GSELB). The charge at SELB is then transferred to thebit line, similar to as described for FIG. 13.

The circuit of FIG. 16 also includes a comparator 720 which compares thevoltage at GSELB with a reference Vref. When the comparator senses thedischarge of the data bus and bit line, it concludes that a SET hassuccessfully occurred and outputs a SET detection signal indicating thatthe memory cell has been SET. The output of comparator 720 is providedto the control logic for the memory system.

FIG. 17 is a timing diagram that explains various embodiments foroperating the circuit of FIG. 16. Between t1 and t2, a pulse is appliedto transistor 706 by the signal XSA_ENABLE. This charges GSELB without acurrent limit, as depicted. Between t3 and t4, a pulse is applied totransistor 710 by the signal XPG_PULSE. This cause the charge to beshared with SELB. The BL Selection circuit allows that charge to beshared with the bit line, as depicted in FIG. 17. In some cases, thisone iteration will cause the memory cell to be SET. In otherembodiments, multiple iterations of the two pulses (charging GSELB andcharge sharing) will be used to increase the charge on the bit lineuntil the memory cell is set (see t5).

FIG. 18 is a schematic diagram of another embodiment of a circuit thatcan be used to set a memory cell using the capacitive dischargedescribed above. In some embodiments, there will be one such circuit foreach bit line or a group of such circuits that can be selectivelyconnected to different groups of bit lines. In the circuit of FIG. 18,the bit line selection device is turned off before the memory cell isswitched into the new state.

The circuit of FIG. 18 includes a memory cell 750 that comprises areversible resistance-switching element and a diode, as described abovewith respect to FIGS. 1-5. Memory cell 750 is connected to a bit line BLhaving a capacitance 752. The bit line BL is connected to a Data bus viathe BL Selection Circuit. In one embodiment, the each bit line has itsown BL selection circuit, and many bit lines can be connected to amulti-line Data bus.

The Data bus, which includes a capacitance 766, is connected to node GSBvia transistor 764, which has its gate connected to ground. Node GSB isconnected to comparator 780, which operates like comparator 720 of FIG.16. The output of comparator 780 is provided to the control logic forthe memory system. Transistor 760, which is connected to VPP and to GSB,receives a pulse (PG Pulse). During a pulse, GSB is floating. Betweenpulses, VPP is used to charge GSB, which charges the Data bus. Based onthe selection signals XCQ<3:0> and “decoder out” the BL SelectionCircuit shares the charge on the data bus with the selected bit line inorder to SET memory cell 750 as discussed above.

The BL Selection Circuit of FIG. 18 includes transistor 768, transistor770, inverter 772, pass gate 774 and pass gate 776. Circle 778 providethe details (four internal transistors and inverter) of pass gates 774and 776. The pass gates have an input (i), output (o), top node (t) andbottom node (b). If the input (i) is a positive voltage, the output (o)receives the signal at the bottom node (b). If the input (i) is anegative or zero voltage, the output (o) receives the signal at the topnode (t). Pass gate 776 receives PG Pulse (same as what is received bytransistor 760). During a pulse (positive voltage), the appropriate oneof XCQ<3:0>, which is input at the bottom node of pass gate 776, isprovided at the output of pass gate 776 and transferred to the output ofpass gate 774 if “decoder out” is also selecting the bit line with apositive voltage. The appropriate one of XCQ<3:0> will be at Vpg(voltage used to SET) for the selected bit line and at VPP for theunselected bit line. When the gate of transistor 768 receives VPP, itcuts off the bit line from the Data bus. When the gate of transistor 768receives Vpg, it shares the charge on the Data bus with the bit line.Note that the gate voltage (Vpg) of transistor 768 can bet set tocontrol the transient current by trim-options.

Between pulses input to pass gate 776, VPP will be transferred to theoutput of pass gate 776 and to the output of pass gate 774, which isthen provided to the gate of transistor 768 to cut off the bit line fromthe Data bus. If XCQ<3:0> or “decoder out” is also selecting the bitline, then VPP will be passed to the gate of transistor 768 to cut offthe bit line from the Data bus.

FIG. 18A is a flow chart describing one embodiment of the operation thecircuit of FIG. 18. In step 788, the selected word line is pulled toground. In step 790, node GSB and the data bus are charged, as explainedabove by transferring VPP to node GSB between pulses of PG Pulses. Instep 792, the charge on the data bus is shared with the bit line byconnecting the bit line to the data bus using the BL Selection circuit,as described above. In step 794, the bit line is cut off from the databus, thereby floating the bit line. As a result, the bit line dischargesthrough memory cell 750 in step 796. In some embodiments, one iterationof the process of FIG. 18A is enough to SET the memory cell. In otherembodiments, multiple iterations are needed to SET the memory cell (see,for example, the processes of FIG. 17 or FIG. 15).

The circuits of FIGS. 13, 16 and 18 limit the amount of electricalcharge in the SET operation rather then the SET current.

Pulse Reset

In previous embodiments, the reversible resistance-switching element isRESET by applying Vreset and providing for a large current through thereversible resistance-switching element. In memory cells that use adiode as the steering element, it is possible during such a RESEToperation to experience some oscillation between SET and RESET or failto provide a sufficiently large current. One solution proposed herein isto perform a RESET by applying a voltage equal to or higher then the SETvoltage for a short pulse time (on the order of tens of nano seconds).The pulse is shorter than necessary for a SET operation, but long enoughfor a RESET operation or a RESET operation broken into multiple pulses.This guarantees that no SET operation can occur and, therefore, nooscillating between SET and RESET. After applying the short pulse, thememory cell can be verified to see whether it has been RESET. If not,another pulse can be applied. This process can repeat until the memorycell is RESET. In one embodiment, the diode is forward biased during theRESET and only positive voltages are used.

FIG. 19 provides one embodiment of a circuit that can perform a RESETusing the short pulses described above. The circuit of FIG. 19 includesa memory cell 800 that comprises a reversible resistance-switchingelement and a diode, as described above with respect to FIGS. 1-5.Memory cell 800 is connected to a bit line BL having a capacitance 802.In one embodiment, capacitance 802 is 1 pf. The bit line BL is connectedto a Data bus via the BL Selection Circuit. In one embodiment, the eachbit line has its own BL selection circuit, and many bit lines can beconnected to a multi-line Data Bus.

The BL selection circuit of FIG. 19 includes transistor 810, transistor816 and inverter 814. Inverter 814 receives an appropriate one ofselection signals CSG<15:0> at its input. In one embodiment, CSG<15:0>is a 16 bit bus coming from a decoder. The top power input to inverter814 receives a short pulse P from a memory system control circuitry.This pulse regulates and causes the short RESET pulse discussed above.During that pulse P, the inverted value of the appropriate one ofselection signals CSG<15:0> is provided at the output (XCSEL) ofinverter 814 and provided to the gate of transistors 810 and 816.Therefore, if the bit line is selected, 0 volts will be applied to thegate of transistors 810 and 816 during a pulse P. If the bit line is notselected, VPP will be applied to the gate of transistors 810 and 816during a pulse P. Between pulses, VPP will be provided the gate oftransistors 810 and 816. When 0 volts is applied to the gate oftransistor 810, the bit line BL will be in communication with the Databus via transistor 810. When VPP is applied to the gate of transistors810 and 816, the unselected bit line voltage UBL will be applied to thebit line via transistor 816. In one embodiment, UBL is ground.

The Data bus is connected to capacitance 806 and transistor 804. Whenthe Data_bit_ENABLE signal applied to the gate of transistor 804 is low(enabled), then VPP is provided to the Data bus via transistor 804.Therefore, when transistor 810 allows the Data bus to communicate withthe bit line, the bit line will be at VPP. When transistor 810 cuts offthe bit line from the data bus, the bit line will be pulled to 0 voltsby device 816. As such, the bit line will see a short pulse equal induration, but opposite in polarity, to the pulse P. The controlcircuitry will provide the pulse P so that it is too short to cause aSET. One or more pulses should cause a RESET.

FIG. 20 is a flow chart describing one embodiment of a process foroperating the circuit of FIG. 19. In step 830, the selected word line ispulled to ground. The unselected word lines are held at Vpp minus 0.7Volts. In step 832, the Data bus is selected and pulled to VPP byappropriately asserting Data_bit_Enable. The bit lines all remain at alow voltage (e.g., 0 volts). In step 834, the bit line is connected tothe Data bus for a short pulse which is applied though the BL SelectionCircuit, as described above. This short pulse may cause a RESET, but itwill not cause a SET. In step 836, a verify operation is performed thatsenses the resistance of the memory cell to detect whether a RESEToccurred. For example, a voltage less than Vreset is applied and thecurrent through the memory cell is measured to determined if the memorycell is in the high-resistivity state or the low-resistivity state. Ifthe memory cell is not yet in the RESET state (step 838), then theprocess loops back to step 834 and another pulse is applied. If thememory cell is verified to have been RESET, then the bit line isunselected in step 840 so that the memory cell 850 does not undergoanother RESET operation.

The process of FIG. 20 uses a verification step between pulses. Thisverification step slows down the RESET process. FIG. 21 is a schematicdiagram of a circuit that performs the RESET process using a shortpulse, but does not use a separate verification step; therefore,increasing the speed of the RESET process.

The circuit of FIG. 21 includes a memory cell 850 that comprises areversible resistance-switching element and a diode, as described abovewith respect to FIGS. 1-5. Memory cell 850 is connected to a bit line BLhaving a capacitance 852. In one embodiment, capacitance 852 is 1 pf.The bit line BL is connected to a Data bus via the BL Selection Circuit.In one embodiment, the each bit line has its own BL selection circuit,and many Bit lines can be connected to a multi-line Data Bus. The BLselection circuit of FIG. 21 is identical to the bit line selectioncircuit of FIG. 19. The data bus includes a capacitance 858 (e.g., 2pf).

The Data bus is connected to transistor 856. The gate of transistor 856is biased at Vread-Vth (approximately 3 volts) so that current flowsbetween the Data bus and node A. Transistor 854 operates similar totransistor 804 of FIG. 19. Transistor 854 receives the signal SA_ENABLEat its gate and, in response to SA_ENABLE, provides Vread (approximately4 volts) to node A.

During a pulse on the bit line, the memory cell experiences Vread. Ifthe memory cell is conducting, it is in the low-resistivity state andthe voltage on the Data bus and at node A drop. This drop in voltagewill be detected by comparator 860, which compares the voltage at node Ato reference voltage Vref. When the memory cell RESETS to thehigh-resistivity state, the memory cell will stop conducting and thevoltage will rise. This rise in voltage will be detected by comparator860. The output of comparator 860 thereby provides a status of thememory cell during the pulse. The control logic for the memory systemcan keep track of which memory cells being RESET in parallel haveachieved the RESET and then unselect them. As such, no separateverification step is needed.

FIG. 21A is a flow chart describing one embodiment of a process foroperating the circuit of FIG. 21. In step 870, the selected word line ispulled to ground. In step 872, the Data bus is selected and pulled toVread by appropriately asserting Data_bit_Enable. The bit lines allremain at a low voltage (e.g., 0 volts). In step 874, the selected bitline is connected to the Data bus for a short pulse which is appliedthough the BL Selection Circuit, as described above. This short pulsemay cause a RESET, but it will not cause a SET. During the short pulseof step 874, the current through the memory cell is sensed and anindication of that sensing is provided to the control logic for thememory system. If the sensing during the pulse detected that a RESEToccurred, then control logic unselects the bit line so that the memorycell 850 does not undergo another RESET operation (step 878).

In some embodiments, after a predetermined number of iterations of theprocess of FIG. 21A that apply the predetermined number of pulses, ifthe memory cell has not been RESET then system control logic 330 willconclude that the memory cell is stuck or otherwise defective. In thatcase, the memory cell is replaced by a redundant memory cell. A datastructure can maintain a correlation between defective memory cells andreplacement memory cells. U.S. Pat. No. 6,868,022, incorporated hereinby reference in its entirety, describes a set of embodiments forproviding and using redundant memory cells to replace defective memorycells.

In some embodiments, the RESET operation described above is performed onmultiple memory cells in parallel. For example, eight or more memorycells may be RESET concurrently. When a particular memory cell isdetected to have been properly RESET, system control logic 330 (oranother circuit utilized in the RESET process) will store (in a latch orother storage device) an indication that the particular memory cell hasbeen RESET so that it will not be subjected to additional RESEToperations.

One embodiment that uses the above-described scheme for performing aRESET can be combined with a system for performing a SET that includesapplying a long SET pulse with a rising voltage level to the memorycell. For example, FIG. 22 depicts a pulse 880 with a rising voltagelevel (labeled as Vsetramp). The current through the memory cell isdetected during the voltage pulse. When the SET current is detected, thepulse is terminated. For example, point 882 indicates when the memorycell was SET. At that time the current spikes (see curve 886), which isindicative of the memory cell entering the low resistivity state. Thevoltage for the memory cell that was SET will initially drop, thenalmost flatten (while the SET is being detected), and subsequently fallto zero volts as the pulse (for that memory cell) is terminated, asdepicted by curve 884. In this way, the minimum voltage level for a SETis applied. Since the diode in the memory cell limits the current and isvery dependent on the SET voltage pulse height, the minimum currentduring SET flows through the memory cell.

The circuit of FIG. 21 can be used, with additional components, toachieve the SET operation discussed with respect to FIG. 22. FIG. 22Ashows a portion (components 810, 814, 816, 850, 852, 858 and 856) of thecircuit of FIG. 21 with additional components 890, 892, 894 and 896.Transistor 856, which has its gate at ground, is connected to comparator890. The other input of comparator 890 is V_(REF), which rampsproportional to Vsetramp. The output of comparator 890, which indicateswhether a SET has been detected, is reported to circuit 896, whichgenerates a reference current Iref for a current mirror. The currentmirror includes pmos transistors 892 and 894, both having their sourceconnected to Vsetramp. The current through transistor 892 mirrors Iref.

In operation, the selected word line WL is pulled down to ground.Vsetramp (the long SET pulse with a rising voltage level) is applied tothe current mirror, as discussed above. The long SET pulse with a risingvoltage level (Vsetramp) is provided to the Data bus from the currentmirror. The bit line BL is connected to the Data Bus for the long pulse,using the BL Selection Circuit. During the pulse, the current is sensedby comparator 890. The current spike 886 will be detected by comparator860 and an indication will be sent to Iref circuit 896 and SystemControl Logic 330. In response receiving the indication that the memorycell has been SET, Iref circuit 896 will stop providing Iref to thecurrent mirror and, instead, will provide 0 amps (or a very smallcurrent) in order to stop the voltage pulse from being provided to thememory cell. In some embodiments, System Control Logic 330 willterminate the pulse (Vsetramp) in response to the indication that thememory cell has been SET. More information about sensing the memory cellduring the programming voltage and stopping programming when sensing thechange in state can be found in U.S. Pat. No. 6,574,145, incorporatedherein by reference in its entirety.

Smart Detection of SET and RESET

As discussed above, during a SET it is possible for the reversibleresistance-switching element to be over-SET so that it then RESETS oroscillates between SET and RESET. Similarly, during a RESET it ispossible for the reversible resistance-switching element to beover-RESET so that it then SETS or oscillates between SET and RESET.Another proposed solution it to test, in real-time, for the reversibleresistance-switching element to RESET (or SET), and then very quicklystop the programming process before the opposite operation oroscillation begins.

FIG. 23 is a circuit that provides for fast detection of RESET and SEToperations. The circuit depicts a memory cell 950 that comprises areversible resistance-switching element and a diode, as described abovewith respect to FIGS. 1-5. Memory cell 950 is connected to bit line BLwhich is driven by bit line driver 952 in response to a Column Selectsignal from the column control circuitry. A voltage is provided to thedriver 952 from transistor 954. FIG. 23 shows transistor 954 drivingvoltage VWR-Vt to the bit line, where VWR is the write voltage and Vt isthe threshold voltage of transistor 954. When performing a RESEToperation, VWR-Vt is a voltage to RESET the reversibleresistance-switching element, such as Vreset (see FIG. 7). Whenperforming a SET operation, VWR-Vt is a voltage to SET the reversibleresistance-switching element, such as Vset (see FIG. 7).

The detection circuitry of FIG. 23 includes two current mirrors. Thefirst current mirror includes transistors 954 and 956. The current atnode X represents the current through the bit line BL when the bit lineis selected. The current at node Y mirrors the current at node X. Asecond current mirror includes transistor 958 and transistor 960.Transistor 960 receives a reference current I_(REFDET) from a circuit inthe system control logic. The current through transistor 958 mirrorsI_(REFDET). Transistor 958 is connected to transistor 956 at a nodelabeled as Fight; therefore, the two current mirrors are connected atnode Fight. Since the terminals of the current mirrors that areconnected together are the mirroring terminals (as opposed to theterminals being mirrored) these connected terminals from the two currentmirrors could try to act differently and, thus, the connecting node islabeled as Fight. If the current out of the first current mirror at nodeX is higher then I_(REFDET), then the voltage at Fight will become high.If the current out of the first current mirror at node X is lower thenI_(REFDET), then the voltage at Fight will become low. The voltage atFight is provided to inverter 962. The output of inverter 962 isprovided to AND gate 966 and an inverted input of AND gate 964. Theother input of AND gate 966 is a signal from the system control logiclabeled RST_MODE, which is asserted high when the circuit of FIG. 23 isattempting to RESET the reversible resistance-switching element and lowotherwise. The other input of AND gate 964 is a signal from the systemcontrol logic labeled SET_MODE, which is asserted high when the circuitof FIG. 23 is attempting to SET the reversible resistance-switchingelement and low otherwise. The outputs of AND gates 964 and 966 areprovided to OR gate 968. The output of OR gate 968 is provided totransistor 940, which when turned on will bring the bit line down toground via node GYSELB.

Note that the circuit of FIG. 23 is depicted for one bit line and onememory cell. It is contemplated that the memory system would havemultiple circuits like the circuit depicted in FIG. 23 so that SET orRESET can be performed concurrently for multiple bit lines or formultiple memory cells.

FIG. 24A is a flow chart describing one embodiment of a process foroperating the circuit of FIG. 23 during a RESET operation. In step 974,the signal RST_MODE is set to logic 1 and SET_MODE is set to logic 0. Instep 976, the column control circuitry applies the appropriate controlsignals to the bit line driver 952. In step 978, VWR is set to the RESETvoltage (e.g., Vreset of FIG. 7). Steps 974 and 978 are performed at thedirection of the system control logic (see FIG. 6). In step 980, the bitline remains charged for the RESET operation to be performed. Prior tothe RESET operation being successful, the reversibleresistance-switching element is in the low-resistivity state; therefore,a high current flows through the memory cell. As a result, the currentat node Y is higher than I_(REFDET) and the voltage at Fight will behigh and the output of inverter 962 will be low. The output of AND gate966 and the output of AND gate 964 will be low; therefore, the output ofOR gate 968 is low and transistor 940 remains off.

In step 982, the RESET occurs and the reversible resistance-switchingelement enters the high-resistivity state. Immediately, in step 984, theRESET operation is stopped. Because the reversible resistance-switchingelement is in the high-resistivity state, the current through the memorycells becomes low which causes the current at node Y to be low. Becausethe current at bode Y is now lower than I_(REFDET), the voltage at Fightwill be low and the output of inverter 962 will be high. The output ofAND gate 966 will be high; therefore, the output of OR gate 968 isbecomes high and transistor 940 is turned on. Once current can flowthrough transistor 960, the bit line will dissipate through transistor940 to ground (via GYSELB), which stops the RESET operation becausethere is not enough voltage differential across the reversibleresistance-switching element.

FIG. 24B is a flow chart describing one embodiment of a process foroperating the circuit of FIG. 23 during a SET operation. In step 988,the signal RST_MODE is set to logic 0 and SET_MODE is set to logic 1. Instep 990, the column control circuitry applies the appropriate controlsignals to the bit line driver 952. In step 992, VWR is set to the SETvoltage (e.g., Vset of FIG. 7). Steps 988 and 992 are performed at thedirection of the system control logic 330 (see FIG. 6). In step 994, thebit line remains charged for the SET operation to be performed. Prior tothe SET operation being successful, the reversible resistance-switchingelement is in the high-resistivity state; therefore, a low current flowsthrough the memory cell. As a result, the current at node Y is lowerthan I_(REFDET), the voltage at Fight will be low, and the output ofinverter 962 will be high. The output of AND gate 966 and the output ofAND gate 964 will be low; therefore, the output of OR gate 968 is lowand transistor 940 remains off.

In step 996, the SET occurs and the reversible resistance-switchingelement enters the low-resistivity state. Immediately, in step 998, theSET operation is stopped. Because the reversible resistance-switchingelement is in the low-resistivity state, the current through the memorycells becomes high which causes the current at node Y to be high.Because the current at bode Y is now higher than I_(REFDET), the voltageat Fight will be high and the output of inverter 962 will be low. Theoutput of AND gate 964 will be high; therefore, the output of OR gate968 is high and transistor 940 is turned on. Once current can flowthrough transistor 960, the bit line will dissipate through transistor940 to ground (via GYSELB), which stops the SET operation because thereis not enough voltage differential across the reversibleresistance-switching element.

In many of the circuit diagrams described above, the depicted circuitscan be replaced by the dual of these circuits where NMOS and PMOS devicetypes are exchanged and positive voltages are exchanged with negativevoltages.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A storage system, comprising: a reversible resistance-switchingmemory cell; a current limiting circuit; a first control line incommunication with the reversible resistance-switching memory cell; afirst selection circuit in communication with the first control line,the first selection circuit selectively provides a first signal to thereversible resistance-switching memory cell; a second control line incommunication with the reversible resistance-switching memory cell; anda second selection circuit in communication with the second controlline, the second selection circuit selectively connects the secondcontrol line to the current limiting circuit while the first selectioncircuit provides the first signal to reversible resistance-switchingmemory cell to provide a reverse bias to the reversibleresistance-switching memory cell that will set the reversibleresistance-switching memory cell to a low resistance state.
 2. Thestorage system of claim 1, wherein: the current limiting circuitincludes a first node, a current mirror having an output at the firstnode and a comparator in communication with the first node; and thefirst node is in communication with the second control line via thesecond selection circuit.
 3. The storage system of claim 2, wherein:current into the first node from the second control line is indicativeof current through the reversible resistance-switching memory cell; thecurrent mirror operates to mirror a reference current; as current intothe first node from the second control line approaches the referencecurrent a voltage at the first node moves toward a reference voltage;and the comparator compares the voltage at the first node with thereference voltage.
 4. The storage system of claim 3, wherein: thecurrent limiting circuit includes a switch in communication with andcontrolled by an output of the comparator to selectively assert a signalon the second control line to deselect the reversibleresistance-switching memory cell from being subjected to a condition forsetting the reversible resistance-switching memory cell to the lowresistance state.
 5. The storage system of claim 4, wherein: thereversible resistance-switching memory cell is associated with a currentlimit for an operation that sets the reversible resistance-switchingmemory cell to the low resistance state; and the reference current isindicative of the current limit.
 6. The storage system of claim 4,wherein: the reversible resistance-switching memory cell includes adiode and reversible resistance-switching material.
 7. The storagesystem of claim 4, wherein: the reversible resistance-switching memorycell is non-volatile.
 8. The storage system of claim 4, furthercomprising: an additional plurality of memory cells, the reversibleresistance-switching memory cell and the additional plurality of memorycells comprise a monolithic three dimensional memory array.
 9. Thestorage system of claim 1, further comprising: a read circuit incommunication with the reversible resistance-switching memory cell. 10.The storage system of claim 1, wherein: the reversibleresistance-switching memory cell includes a steering device andreversible resistance-switching material.
 11. The storage system ofclaim 10, wherein: the reversible resistance-switching materialcomprises metal oxide; and the steering device is a diode.
 12. A storagesystem, comprising: a non-volatile memory cell; a first control line incommunication with the memory cell; a first selection circuit incommunication with the first control line, the first selection circuitselectively provides a first signal to the memory cell; a second controlline in communication with the memory cell; a second selection circuitin communication with the second control line, the second selectioncircuit selectively provides a second signal to the memory cell whilethe first selection circuit provides the first signal to the memory cellto provide a reverse bias to the memory cell that will set the memorycell to a low resistance state; and a current limiting circuit incommunication with the memory cell.
 13. The storage system of claim 12,wherein: the current limiting circuit includes a first node, a currentmirror having an output at the first node and a comparator incommunication with the first node; and the first node is incommunication with the second control line via the second selectioncircuit.
 14. The storage system of claim 13, wherein: current into thefirst node from the second control line is indicative of current throughthe memory cell; the current mirror operates to mirror a referencecurrent; as current into the first node from the second control lineapproaches the reference current a voltage at the first node movestoward a reference voltage; and the comparator compares the voltage atthe first node with the reference voltage.
 15. The storage system ofclaim 12, wherein: the current limiting circuit includes a switch toselectively assert a signal on the second control line to deselect thememory cell from being subjected to a condition for setting the memorycell to the low resistance state.
 16. The storage system of claim 12,wherein: the reversible resistance-switching memory cell includes adiode and metal oxide material.
 17. A method for operating a storagesystem, comprising: setting a reversible resistance-switchingnonvolatile storage memory cell to a low resistance state by reversebiasing the reversible resistance-switching nonvolatile storage memorycell; and limiting current through the reversible resistance-switchingnonvolatile storage memory cell using a current limiting circuit whilesetting the reversible resistance-switching nonvolatile storage memorycell to the low resistance state.
 18. The method of claim 17, whereinsetting the reversible resistance-switching nonvolatile storage memorycell to the low resistance state comprises: biasing word lines and bitlines for a monolithic three dimensional memory array to a first voltagelevel, the monolithic three dimensional memory array includes thereversible resistance-switching nonvolatile storage memory cell as wellas other reversible resistance-switching storage memory cell; biasing aselected word line to at least a diode drop higher than the firstvoltage level; and connecting a selected bit line to the currentlimiting circuit with a path to ground.
 19. The method of claim 18,wherein limiting current through the reversible resistance-switchingnonvolatile storage memory cell comprises: operating a current mirrorconnected to the selected bit line via a first node; monitoring thevoltage at the first node; comparing the voltage at the first node to areference voltage; and applying a save voltage to the selected bit linewhen the voltage at the first node is at a predetermined level withrespect to the reference voltage.
 20. The method of claim 19, whereinlimiting current through the reversible resistance-switching nonvolatilestorage memory cell further comprises: disabling a reference currentinput for the current mirror when the voltage at the first node is at apredetermined level with respect to the reference voltage.
 21. Themethod of claim 19, wherein: the current mirror includes a referencecurrent input that is mirrored at an output of the current mirror, thereference current input is equal to a current limit for setting thereversible resistance-switching nonvolatile storage memory cell to thelow resistance state.
 22. A method for operating a storage system,comprising: setting a reversible resistance-switching nonvolatilestorage memory cell to a low resistance state including providing afirst signal to a first terminal of the reversible resistance-switchingnonvolatile storage memory cell and providing a second signal to asecond terminal of the reversible resistance-switching nonvolatilestorage memory cell to cause a current through the reversibleresistance-switching nonvolatile storage memory cell in a direction thatis reverse of easiest current flow through the reversibleresistance-switching nonvolatile storage memory cell; and limitingcurrent through the reversible resistance-switching nonvolatile storagememory cell using a current limiting circuit while setting thereversible resistance-switching nonvolatile storage memory cell to thelow resistance state.
 23. The method of claim 22, wherein setting thereversible resistance-switching nonvolatile storage memory cell to thelow resistance state comprises: biasing word lines and bit lines for amonolithic three dimensional memory array to a first voltage level, themonolithic three dimensional memory array includes the reversibleresistance-switching nonvolatile storage memory cell as well as otherreversible resistance-switching storage memory cell; biasing a selectedword line to at least a diode drop higher than the first voltage level;and connecting a selected bit line to the current limiting circuit witha path to ground.
 24. The method of claim 22, wherein limiting currentthrough the reversible resistance-switching nonvolatile storage memorycell comprises: operating a current mirror in communication with thereversible resistance-switching nonvolatile storage memory cell via afirst node; monitoring the voltage at the first node; comparing thevoltage at the first node to a reference voltage; and asserting a signalon the second control line to deselect the reversibleresistance-switching nonvolatile storage memory cell from beingsubjected to a condition for setting to the low resistance state. 25.The method of claim 24, wherein limiting current through the reversibleresistance-switching nonvolatile storage memory cell further comprises:disabling the reference current input for the current mirror when thevoltage at the first node is at a predetermined level with respect tothe reference voltage.
 26. The method of claim 22, wherein setting thereversible resistance-switching nonvolatile storage memory cell to thelow resistance state comprises: biasing unselected word lines andunselected bit lines to ground; biasing a selected word line to one sideof ground; and providing a selected bit line with a path to a voltage atthe other side of ground.